Antigenic variation has previously been documented in vitro using the WB isolate of Giardia lamblia. Similar phenomena occurred in two other isolates indicating antigenic variation occurs generally in Giardia and is not limited to specific isolates. Humans and gerbils were inoculated with defined clones of giardia and subsequent isolates studies. Both reveals loss of the major surface antigens and replacement with other antigens demonstrating antigenic variation occurs in humans and gerbils. Analysis of humoral responses revealed surface and isolate specific responses. Ultrasonography was used to study the presence of Symmers' fibrosis in treated and controlled populations compared to untreated villages. A marked decrease of Symmers' fibrosis was found in the treated villages showing treatment of schistosomiasis resulted in a marked decrease in morbidity and mortality. The need for humoral antibodies in the treatment of cysticercosis with praziquantel was studied in T. taeniaformis infections in mice. Mu treated mice were unable to cure their infection after praziquantel treatment compared to competent mice.